The High definition television (HDTV) broadcast standard adopted by the Advanced Television Systems Committee (ATSC) in the U.S. and originally proposed by the “Grand Alliance” of several television manufacturers and research laboratories is described in the “ATSC Digital Television Standard”, Document A/53 published on Sep. 16, 1995. This document sets forth all the requirements regarding HDTV signal characteristics. In particular, the ATSC-HDTV signals require a vestigial sideband modulation format.
Vestigial sideband (VSB) modulation is a well-known modulation method for digitally transmitting data such as HDTV signals. Recovery of data by a digital receiver from the transmitted ATSC-VSB signal, which contains the digital video, audio and related information, inherently requires the implementation of several functions, including but not limited to: timing recovery for symbol synchronization, carrier recovery or tracking, and equalization.
Timing recovery is the process by which the receiver clock (timebase) is synchronized to the transmitter clock by decoding the timing signal which is embedded in the transmitted VSB signal. The two quantities which the receiver must determine in order to achieve symbol synchronization are the sampling frequency and the sampling phase. The sampling frequency is typically specified but oscillator drift will introduce deviations from the specified symbol rate. The sampling phase is the correct time within a symbol period during which to take a symbol representative data sample.
Real world symbol pulse shapes have a peak in the center of the symbol period. Sampling the symbol at this peak will produce the best signal-to-noise ratio (SNR) and will ideally eliminate intersymbol interference (ISI) associated with the pulse shape. An example of a symbol timing recovery loop (STL) is disclosed in U.S. Pat. No. 5,943,369, entitled TIMING RECOVERY SYSTEM FOR A DIGITAL SIGNAL PROCESSOR, issued Aug. 24, 1999 to Knutson et al. The accuracy of the recovered timing signal is substantially equivalent to the accuracy of the transmitted VSB timing signal.
Carrier tracking addresses the problem of frequency offset between the transmitter and receiver oscillators either intrinsic to their design or due to drifts, causing a drift of the sinusoidal carrier signal. At the transmitter, the carrier signal is multiplied by a data representative signal in order to modulate the carrier signal to a passband radio frequency (RF) center frequency. At the receiver the passband RF signal is demodulated to a lower intermediate frequency (IF) or directly to baseband by being multiplied by a sinusoid generated by the local oscillator (LO). Ideally the LO and transmitter oscillator frequencies will match, but in practice any difference will result in the demodulated signal being not at but near the IF center frequency or baseband with some frequency offset. The offset will cause the received signal constellation to rotate and forbid signal recovery. This “spinning” effect must be removed before accurate symbol decisions can be made. The purpose of the carrier tracking loop (CTL) is to remove this frequency offset and demodulate the signal down to baseband (from the original IF or near baseband frequency), so that the received signal can be processed accurately directly at baseband. In the case of the VSB transmitted signal, by frequency shifting the signal down to baseband accurately the full signal spectrum can be recovered from the RF spectrum, since only a portion of the original spectrum is actually transmitted.
Equalization is a signal processing technique that attempts to correct for linear distortions in the received signal, which appear in the form of ISI, mainly caused by channel impairments (e.g., multipath propagation in the terrestrial broadcast channel) or by filtering in the receiver or transmitter.
Practical receivers maximize the SNR at the receiver by using a technique known as matched filtering. A receiver employing such a technique filters the received signal with a filter whose shape is “matched” to the transmitted signal's pulse shape. The output of the filter is then sampled at times nT; where T is the symbol time interval. The matched filter's pulse shape is a time-reversed version of the transmitted pulse shape. Such processing has two advantages. One advantage is that typical pulse shapes have a low pass response. By filtering the received signal with a low pass filter, the frequencies containing the data signal are passed while the remaining higher frequencies, containing only noise, are attenuated. Matched filtering thus limits the amount of the RF spectrum that must be processed by subsequent stages of the receiver. A second advantage is that a matched filter essentially correlates the received signal with the transmitted pulse shape over the time period T when the symbol is present.
The ideal signal pulse shape should have a limited bandwidth and zero ISI. One ideal pulse shape that meets these requirements is a time domain sinc pulse. All pulses of limited bandwidth, like the sinc pulse, are infinite in time duration. Practical implementations require that these pulses, or specifically the filters that create these pulses, be truncated to a finite length. The pulse or filter length is chosen according to a specified level of reliability. This reliability is associated with the amount of ISI in the signal, which has to be kept to a minimum. The spectrum of the sinc pulse has the shape of a rectangle, which implies a sharp discontinuity in the frequency domain. In order to restrict the amount of ISI, the length of the truncated pulse must be long. One pulse shape that has properties similar to the sinc pulse but without the frequency domain discontinuities is the raised cosine pulse. Because of its smoother spectrum, it is easier to truncate such pulses in the time domain. The most popular pulse shape used in practical communications systems is the root raised cosine pulse, which is formed by taking the square root of the spectrum of a raised cosine pulse. This pulse shape filter is used in both the transmitter and the receiver in order to split the spectral characteristics of the raised cosine pulse equally between the transmitter and the receiver. By cascading two root raised cosine filters together (one filter in the transmitter and the other matched filter in the receiver), the root raised cosine pulse spectrum is squared, thus creating a net system response of the desired raised cosine pulse. The output of the matched filter at the receiver is then sampled at symbol times nT.
A typical HDTV receiver demodulating scheme is disclosed in U.S. Pat. No. 6,233,295 entitled SEGMENT SYNC RECOVERY NETWORK FOR AN HDTV RECEIVER issued on May 15, 2001 to Wang. The disclosed demodulator includes an analog to digital (A/D) converter sampling at 21.52 MHz, a carrier tracking loop (CTL) operating at 21.52 MHz, followed by a symbol timing loop (STL) producing symbol samples at 10.76 MHz, followed by a sync detector, equalizer and phase tracker, all operating at the symbol rate of 10.76 MHz.
According to the ATSC-HDTV standard the IF input to the demodulator is a root raised cosine filtered signal (filtered in the transmitter), which should be matched by a similar root raised cosine filter in the receiver in order to avoid linear distortion and ISI due to the pulse shape. The receiver matched filter can be placed prior to the A/D as an analog filter, or before the CTL as a digital filter. However, the matched filter may also be placed after the CTL or even after the STL as a digital filter. In fact, because the equalizer is a digital filter, in theory, it can provide both channel equalization and matched filtering. In these cases, the signal processed by the CTL and the STL will not have been matched filtered, if both functions precede the equalizer.
Therefore, in the design of a general purpose HDTV demodulator subsystem, possibly in the form of an integrated circuit chip, several possibilities must be accommodated. The CTL input signal will be rotating in phase and the input signal may be either matched, i.e. fully raised cosine filtered by a preceding root raised cosine filter; or unmatched, i.e. the matching root raised cosine filter following the CTL. This along with the placement of the CTL prior to the STL implies that the CTL signal level may be up to 1.7 (matched case) or 2.0 (unmatched case) times the 8-VSB slicer levels. This need for a higher dynamic range in the CTL should be taken into consideration, otherwise, this condition can result in overflow in the CTL, which introduces severe nonlinearities in the demodulation process.
A demodulator design is desirable, which can compensate for the various possible placements for the receiver matching filter, as well as for different signal dynamic ranges in different demodulator blocks.